The present invention relates generally to carriers for chip-scale devices, also referred to as wafer scale packaging (WSP) devices or as WSP chips, and also relates to techniques for rapid, efficient thermal testing and/or thermal cycling of WSP chips.
Thermal testing and/or cycling of a batch of WSP chips ordinarily is accomplished by placing a large number of WSP chips in a conventional plastic carrier, placing the carrier in a thermal chamber, and either heating the chamber and/or passing a heated gas or liquid medium through the chamber. For temperature cycling, typically the carrier and the WSP chips therein are alternately subjected to “hot baths” and “cold baths” of gas or liquid medium to provide rapid thermal ramp-up times and thermal ramp-down times. A typical liquid used for this purpose is “FLUORINERT”, which is commercially available from 3M Corporation. A typical inert gas used as a thermal medium is nitrogen.
One prior art chip carrier, part number H20-130-2462-C02 available from Entregris Corporation, is shown in FIG. 1.
The Entregris chip carrier product of FIG. 1 has the shortcoming that it does not allow fluid thermal medium to flow through the carrier and come in direct contact with the chips being carried. The Entregris chip carrier therefore has very long thermal ramp-up and ramp-down times, which adds substantially to the cost of thermal stress cycling procedures. Typically, five-minute temperature ramping times or less are desirable in thermal cycling, between, for example, −55 degrees Celsius (C.°) to +125 C.° or even as high as +150 C.°. Another shortcoming of the Entregris chip carrier product of FIG. 1 is that the plastic material, which is manufactured under the trade mark FLUOROWARE, does not tolerate high temperatures. Another shortcoming is that the plastic material out-gases at temperatures slightly above room temperature, which may deleteriously affect the performance of chips in the carrier. The plastic is composed of carbon-impregnated petro-chemical materials, and the plastic usually is coated by a layer of anti-static material. Consequently, heating the plastic carrier results in release of free ionic gases. The out-gassing tends to cause electronic charge and plastic residues to be deposited on the chip surfaces. This often causes errors in circuit operation of the chips, resulting in loss of the chips during functional testing thereof.
Other conventional chip carriers typically are also made of plastic material. None of the unknown chip carriers are well-suited for supporting WSP chips during the thermal testing and/or thermal cycling that usually is a requirement for a semiconductor manufacturer to meet the “qualification” standards for each product that most large customers require to be met before they will purchase the product.
There are additional reasons that cause conventional fixturing mechanisms and devices, such as the above described Entregris chip carrier, to be unsuitable for performing thermal stress test sequences and thermal cycling on small devices such as WSP chips. Presently available fixturing mechanisms such as chip support trays do not adequately support WSP chips under test, and do not allow proper flow of gas or liquid thermal mediums around the WSP chips to be thermally tested or thermally cycled.
Also, the thermal mass of the prior art chip support fixturing devices or trays is so large that it greatly reduces the rate at which the WSP chips attain the desired temperatures. This has prevented the desired amount of thermal shock specified by the above-mentioned qualification standards from being applied to the WSP chips, because most of the thermal energy from the thermal medium is being transferred between the thermal medium and the prior art carriers, rather than between the thermal medium and the chips. Furthermore, most of the thermal energy involved in the thermal cycling, has been wasted.
Also, the prior art plastic chip carriers tend to warp or be physically deformed due to mismatches in temperature expansion coefficients of the materials, and the resulting stretching, flexing, etc. of the materials when subjected to increased temperatures may interfere with the ability of the carriers to adequately hold the WSP chips, and may displace them from the carrier cavities in which the WSP chips are intended to be supported. Such displacement of a WSP chip may result in damage to it while it is in a thermal testing or thermal cycling chamber. The damage may include chipping of edges of the chip and/or damage to the chip metallization (especially to solder bumps that are used for external electrical contact to the chip metallization), causing rejection and loss of the chip at the functional testing stage.
Thus, there is an unmet need for a fixturing mechanism capable of reliably containing and supporting WSP chips and like to be tested, wherein the fixturing mechanism allows a thermal gas or liquid medium to readily and uniformly flow around the WSP chips under test.
There also is an unmet need for a thermal stress fixture that does not damage WSP chips therein.
There also is an unmet need for a thermal stress fixture that allows fast temperature ramp-up and fast temperature ramp-down during thermal stress cycling.
There also is an unmet need for a thermal stress fixture that avoids waste of thermal energy during thermal stress testing and/or thermal cycling.
There also is an unmet need for a thermal stress fixture that avoids damage to semiconductor chips due to out-gassing of substances from materials of which the thermal stress fixture is composed.